With a humanitarian crisis unfolding on the U.S.-Mexico border, President Obama has announced that he intends to ask for $2 billion in emergency funding, but then bypass an uncooperative Congress and act through executive orders to address the situation. The politics of immigration are admittedly hard, as Representative Eric Cantor painfully learned during his primary race several weeks ago, but the economics are easy. The debate today focuses on humanitarian issues, the concept of fairness, the appropriate legal framework and how to integrate immigrants – whether legal or not – into American society. The long-term economic implications deserve to be at the center of this discussion.

Lost amidst the national debate on immigration reform is the critical fact that we need to attract workers from abroad to maintain the long term growth of our labor force. U.S. fertility rates are right at population replacement levels, and labor force participation has been in decline since the 1970s. Thus, without immigration, we are headed toward a stagnant or even declining labor force in the not too distant future, with dire implications for economic growth. Getting immigration right is an economic imperative for the 21st century.

Data from the Population Division of the United Nations paints a stark picture of what the future labor force of the United States would look like without immigration. Using projected fertility rates and current immigration trends, the population of the United States continues to grow throughout the 21st century, topping 460 million inhabitants at the turn of the 22nd century. If we subtract immigration from the data, the population of the United States stops growing in about 2042, with seriously negative implications for economic activity.

Why is the labor force so important? In the short run, economic activity is determined by demand: the degree to which people consume, businesses invest and governments tax and spend. In the long run, however, economic activity is driven by the supply of resources, and the most critical resource turns out to be human capital. The capacity of any economy to expand over time is a function of growth in the labor force plus the productivity of that labor force.

A few simple dynamics determine growth in the labor force: fertility, mortality, participation, and immigration. We routinely think of these as demographic or social issues, but the economic implications are meaningful. Our economic capacity is defined by how many new people are being born, how long they live, the extent to which they participate in the formal economy, and whether or not we can “borrow” population growth from other countries through immigration.

On several of these measures, the United States is in relatively good demographic shape compared to the rest of the world. Our fertility rate of 2.06 births per woman is right in line with population replacement levels, but better than Germany, Japan, France, Brazil and even China. Our health care system, although beset with well-documented challenges, has nevertheless resulted in longer, healthier and more productive lifespans than ever before. Aging workers pose their own challenges to government policy and corporate protocols, but an information or knowledge based economy ought to benefit from retaining experienced workers, whose human capital does not deteriorate over time, but increases.

Yet the demographic front has bad news, too. The labor force participation rate in the United States in May stood at 62.8%, a 36-year low. This decline in participation may reflect the leading edge of baby boomer retirements, or lingering effects of the financial crisis, or even a shift in how we define work. But the absence of 29.2% of the working age population from the labor force is an undeniable drag on economic potential.

Net immigration helps to offset this drag, and furthermore provides a boost to population and labor force growth if fertility rates continue to decline. A survey of global economies provides plenty of examples of economies that struggle with stagnant or even negative population or labor force growth, and illustrates how difficult it is to generate sustainable economic growth when human capital is in short supply. Japan – an economy relatively closed to immigration – stands as the clearest example of these challenges.

In addition to boosting absolute population or labor force levels, immigration provides a further demographic benefit in that immigrants tend to be younger, and bring with them higher fertility rates. This is admittedly a provocative dynamic. There are those who bemoan the literally changing face of America, but they should balance those concerns against the economically unattractive alternative of a stagnant population and meager economic growth.

Congressional Republicans clearly do not trust the White House to enforce immigration reform, and Speaker Boehner has carried that message to the Oval Office. Eric Cantor’s primary loss sends a sobering message to other Republicans who have expressed a desire to work with Democrats to fix a broken system. That is unfortunate. Immigration is clearly an emotional topic for many Americans, but the fact that people risk their lives to come to America is a sign of national strength, not weakness. We have to get immigration right.

The difference between a functional and dysfunctional immigration policy is the difference between a dynamic economic future or one mired in malaise. Our economy needs human capital, whether we grow it at home or welcome it from abroad. What’s happening on our southern borders is a humanitarian crisis at present, but could become a chronic economic crisis if we don’t create a solution that provides a path to citizenship for the men, women and children who are risking their lives to come to this country. Some call that amnesty. I call it sound long-term economic policy.